Samenvatting
Recent advances in hybrid computational aeroacoustics (CAA) have enabled wide application of numerical methods for noise reduction studies in aeronautical applications. Since in realistic configurations noise reduction at the source may compromise aerodynamic performance, other alternative techniques are sought. Advanced flow control techniques and effective shielding or redirection of scattered noise by the airframe can positively alter an aircraft’s acoustic signature. Numerical tools based on hybrid CAA can add efficiency to the design process when novel noise reduction techniques are introduced. This dissertation contributes to the state of the art of hybrid CAA studies by introducing flow control techniques for noise reduction in aeronautical applications and by developing efficient integral methods for acoustic scattering prediction.In the frame of noise control techniques, a rotating cylinder was initially examined individually, and subsequently as part of a rudimentary airframe configuration, namely the rod-airfoil configuration. Hybrid CAA was used for noise prediction, where acoustic analogy formulations were used for noise propagation. Noise source identification was performed by unsteady Reynolds-averaged Navier-Stokes computations and by large-eddy simulation (LES), in order to predict tonal and broadband noise emissions, respectively. The fast and efficient rotation-rate based Smagorinsky subgrid scale model was used for LES. Aerodynamic results showed that cylinder rotation leads to the suppression of the vortex shedding street and its deflection away from the symmetry plane. As a result, generally increasing lift forces and decreasing drag forces are observed for the rod and the airfoil. These phenomena become more prominent with increasing frequency of rotation. Cylinder rotation proved to be advantageous for noise control purposes, since increasing rotational frequency resulted in notable tonal noise reduction for both the rod and the airfoil. This reduction was attributed to the vortex shedding suppression. Broadband noise emissions showed slight increase for the rod, but notable decrease for the airfoil.
Regarding acoustic scattering predictions, numerical methodologies were suggested based on the equivalent source method (ESM) for scattering computations in the time and in the frequency domain. The ESM was chosen due to its simplicity and efficiency. Integral formulations based on the Ffowcs-Williams and Hawkings (FW-H) equation and the Kirchhoff method were used for acoustic propagation.
Firstly, a time-domain ESM for stationary media was suggested and validated. Incident and scattered acoustic predictions are realized by acoustic velocity formulation V1A of Ghorbaniasl. Formulation V1A is based on the FW-H equation and allows direct evaluation of the boundary condition, without resorting to acoustic pressure gradient computation. Formulation V1A is more efficient than existent time-domain acoustic pressure gradient formulations, which implies computational savings during the evaluation of the scattering boundary condition. Additionally, a time-domain scattering approach is more efficient than its frequency-domain counterparts, for broadband noise predictions.
Secondly, a frequency-domain ESM for stationary medium problems was developed. The boundary condition was initially evaluated by acoustic velocity formulations V1A and KV1A of Ghorbaniasl, which allowed visualization of acoustic scattering and propagation characteristics. Formulation KV1A is based on the Kirchhoff method and can be easily implemented in scattering methodologies for high-speed sources, where the Kirchhoff formula has already been coded. Subsequently, frequency-domain acoustic pressure gradient formulations, based on the FW-H equation, were coupled to the ESM in order to suggest an approach based entirely on frequency-domain methods. The methodology was validated and applied for acoustic scattering by a helicopter rotor, thus proving its efficiency for scattering from periodic noise sources. It is additionally applicable to any scattering surface geometry, without restrictions.
Thirdly, a convected frequency-domain ESM was suggested for scattering predictions in a moving medium. Moving-medium acoustic pressure gradient formulations based on the FW-H equation were used to derive convected equivalent sources, which allow evaluation of the scattering boundary condition in a moving medium. The need for a Lorentz transformation is obviated, allowing straightforward scattering predictions in uniform constant flows of any velocity. The methodology is valid for acoustic scattering by slender aerodynamic bodies. The approach was implemented and validated for analytical test cases of periodic and rotating sources in a moving medium.
The outcome of this dissertation comprises of contributions to the state of the art of noise control studies and acoustic scattering prediction, by hybrid CAA. The rotating cylinder showed encouraging noise reduction capabilities, combined with enhanced aerodynamic properties, as part of the rod-airfoil configuration. Moreover, the suggested ESM methodologies for acoustic scattering provided accurate and efficient computations for scattering predictions in stationary and moving media. The suggested noise control technique and noise prediction methodologies display the potential to be incorporated in the design process of aircraft, towards reduced noise emissions.
Datum prijs | 19 jun 2018 |
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Originele taal | English |
Begeleider | Tim De Troyer (Promotor) & Ghader Ghorbaniasl (Promotor) |